Multi-core optical fiber ribbons and methods for making the same

ABSTRACT

Multi-core optical fiber ribbons and methods for making multi-core optical fiber ribbons are described herein. In one embodiment, a multi-core optical fiber ribbon includes at least two core members formed from silica-based glass and oriented in parallel with one another in a single plane. Adjacent core members have a center-to-center spacing ≧15 microns and a cross-talk between adjacent core members is ≦−25 dB. In this embodiment each core member is single-moded with an index of refraction n c , and a core diameter d c . In an alternative embodiment, each core member is multi-moded and the center-to-center spacing between adjacent core members is ≧25 microns. A single cladding layer is formed from silica-based glass and surrounds and is in direct contact with the core members. The single cladding layer is substantially rectangular in cross section with a thickness ≦400 microns and an index of refraction n cl &lt;n c .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/273,495, filed Oct. 14, 2011, which claims the benefit of priorityunder 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/411,017filed on Nov. 8, 2010, both of which are hereby incorporated byreference for all purposes as if fully set forth herein.

BACKGROUND

Field

The present specification generally relates to optical fibers ribbonsand, more specifically, to optical fiber ribbons with multiple cores andmethods for making the same.

Technical Background

In recent years optical fiber has become accepted as a viablealternative to traditional materials used for data signal communication.Optical fiber is now widely utilized in a variety of electronic devicesto facilitate the high-speed communication of data signals at highbandwidths. However, as the speed and bandwidth of the electroniccomponents in data communication devices increases, there is acorresponding need to increase the speed of optical interconnects whichcouple such devices. One solution to increase the speed of opticalinterconnects is to increase the fiber density of the opticalinterconnects and thereby realize high fiber count connectors. However,increasing the number of individual fibers in an optical interconnectadds to the overall size of the optical interconnect.

SUMMARY

According to one embodiment, a multi-core optical fiber ribbon includesat least two core members formed from silica-based glass and oriented inparallel with one another in a single plane. Adjacent core members havea center-to-center spacing ≧15 microns and a cross-talk between adjacentcore members is ≦−25 dB. Each core member is single-moded with an indexof refraction n_(c), and a core diameter d_(c). A single cladding layeris formed from silica-based glass and surrounds and is in direct contactwith the core members. The single cladding layer is substantiallyrectangular in cross section with a thickness ≦400 microns and an indexof refraction n_(cl)<n_(c).

In another embodiment, a multi-core optical fiber ribbon includes atleast two core members formed from silica-based glass and oriented inparallel with one another in a single plane. Adjacent core members havea center-to-center spacing ≧35 microns and a cross-talk between adjacentcore members is ≦−25 dB. Each core member is multi-moded with an indexof refraction n_(c), a diameter d_(c) of greater than 15 microns, has analpha profile with an α value from about 1.9 to about 2.1, and abandwidth >300 MHz. A single cladding layer formed from silica-basedglass surrounds and is in direct contact with the at least two coremembers. The single cladding layer is substantially rectangular in crosssection with a thickness ≦400 microns and a cladding index of refractionn_(cl)<n_(c).

In another embodiment, a method for forming a multi-core optical fiberribbon includes forming a core cane assembly comprising a plurality ofcore cane members oriented in parallel in a single plane. The core caneassembly is positioned in a flow of carrier gas comprising silica-glassprecursor materials. A burner is traversed across a first surface of thecore cane assembly to react the silica-glass precursor materials in theflow of carrier gas thereby causing silica-glass soot to be deposited onthe first surface of the core cane assembly. The burner is traversedacross a second surface of the core cane assembly to react thesilica-glass precursor materials in the flow of carrier gas therebycausing silica-glass soot to be deposited on the second surface of thecore cane assembly. Thereafter, the silica-glass soot on the core caneassembly is consolidated to form a multi-core ribbon preform comprisinga core cane assembly encircled by a single cladding preform layer,wherein the single cladding preform layer is substantially rectangularin cross section. The multi-core ribbon preform is then drawn into amulti-core optical fiber ribbon having a substantially rectangular crosssection.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the embodiments described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description describe various embodiments and areintended to provide an overview or framework for understanding thenature and character of the claimed subject matter. The accompanyingdrawings are included to provide a further understanding of the variousembodiments, and are incorporated into and constitute a part of thisspecification. The drawings illustrate the various embodiments describedherein, and together with the description serve to explain theprinciples and operations of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts a cross section of a multi-core opticalfiber ribbon according to one or more embodiments shown and describedherein;

FIG. 2A schematically depicts a cross section of a multi-core opticalfiber ribbon according to one or more embodiments shown and describedherein;

FIG. 2B schematically depicts a cross section of a core member of themulti-core optical fiber ribbon of claim 2A;

FIG. 3 schematically depicts a cross section of a multi-core opticalfiber ribbon according to one or more embodiments shown and describedherein;

FIG. 4 schematically depicts a core cane assembly for a multi-coreribbon preform according to one or more embodiments shown and describedherein;

FIGS. 5-6 schematically depicts a method for forming a single claddingpreform layer on a core cane assembly according to one or moreembodiments shown and described herein;

FIG. 7 schematically depicts a cross section of a rectangular mold witha core cane assembly for a multi-core optical fiber ribbon positioned ina rectangular mold cavity and surrounded by glass soot;

FIG. 8 schematically depicts a cross section of a rectangular mold asglass soot is compressed around a core cane assembly;

FIG. 9 schematically depicts a multi-core ribbon preform comprising acore cane assembly surrounded by a single cladding preform layeraccording to one or more embodiments shown and described herein;

FIG. 10 schematically depicts one embodiment of a multi-core ribbonpreform comprising a core cane assembly positioned within a rectangularglass cladding tube;

FIG. 11 schematically depicts a multi-core optical fiber ribbon beingdrawn from a multi-core optical fiber preform, according to one or moreembodiments shown and described herein;

FIG. 12 graphically depicts a plot of the cross-talk between twoadjacent core members as a function of the center-to-center spacingbetween the two adjacent core members for i) a pair of identical coremembers and ii) a pair of core members with a delta variation of 1%;

FIG. 13 graphically depicts a plot of the cross-talk between twoadjacent core members as a function of the center-to-center spacingbetween the two adjacent core members for i) a fiber length of 100meters and ii) a fiber length of 2 meters;

FIG. 14 graphically depicts a plot of the of the cross-talk between twoadjacent core members as a function of the center-to-center spacingbetween the two adjacent core members for i) a pair of core members witha core relative refractive index Δ_(c) %=0.34% and ii) a pair of coremembers with a core relative refractive index Δ_(c) %=1.0%; and

FIG. 15 graphically depicts a plot of the of the cross-talk between twoadjacent core members as a function of the center-to-center spacingbetween the two adjacent core members for i) a pair of core membersformed without a low-index ring and ii) a pair of core members formedwith a low-index ring.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments ofmulti-core optical fiber ribbons, examples of which are schematicallyillustrated in the accompanying drawings. Whenever possible, the samereference numerals will be used throughout the drawings to refer to thesame or like parts. One embodiment of an optical fiber ribbon isschematically illustrated in FIG. 1. The multi-core optical fiber ribbongenerally comprises at least two core members formed from silica-basedglass and surrounded by a single cladding layer which is also formedfrom silica-based glass. The core members are oriented in parallel withone another in a single plane and spaced such that the cross-talkbetween adjacent core members is ≦25 dB. The single cladding layer issubstantially rectangular in cross section such that the optical fiberribbon is also substantially rectangular in cross section. Variousembodiments of multi-core optical fiber ribbons and methods for makingmulti-core optical fiber ribbons will be described in more detailherein.

The phrase “refractive index profile,” as used herein, refers to therelationship between refractive index or relative refractive index andthe dimensions of the optical fiber ribbon.

The phrase “relative refractive index,” as used herein, is defined asΔ%=100×(n_(i) ²−n_(REF) ²)/2n_(i) ², where n_(i) is the maximumrefractive index in region i, unless otherwise specified. The relativerefractive index percent is measured at 1300 nm unless otherwisespecified. Unless otherwise specified herein, n_(REF) is the averagerefractive index of the single cladding layer, which can be calculated,for example, by taking “N” index measurements (n_(cl), n_(c2), . . .n_(cN)) of the single cladding layer (which in some preferredembodiments may be undoped silica), and calculating the averagerefractive index by:

$n_{C} = {\left( {1\text{/}N} \right){\sum\limits_{i = 1}^{i = N}\; n_{Ci}}}$

As used herein, the relative refractive index is represented by Δ% andits values are given in units of “%,” unless otherwise specified. Incases where the refractive index of a region is less than the referenceindex n_(REF), the relative index percent is negative and is referred toas having a depressed region or depressed-index, and the minimumrelative refractive index is calculated at the point at which therelative refractive index is most negative unless otherwise specified.In cases where the refractive index of a region is greater than thereference index n_(REF), the relative index percent is positive and theregion can be said to be raised or to have a positive index.

Bandwidth may be measured at 1300 nm (unless another wavelength isspecified) according to FOTP-204 with overfilled launch.

The term “α-profile” or “alpha profile” refers to a relative refractiveindex profile of the core members, expressed in terms of Δ(r) which isin units of “%”, where r is the radius of the core member, which followsthe equation:Δ(r) %=Δ(r _(o))(1−[|r−r _(o)|/(r ₁ −r _(o))]^(α)),where r_(o) is the point at which Δ(r) is maximum, r_(l) is the point atwhich Δ(r) % is zero with respect to the single cladding layer, and r isin the range r_(i)≦r≦r_(f), where Δ is defined above, r_(i) is theinitial point of the α-profile, r_(f) is the final point of theα-profile, and αis an exponent which is a real number. For a profilesegment beginning at the centerline of a core member (r=0), theα-profile has the simpler formΔ(r) %=Δ(0)(1−[|r|/(r _(l)]^(α)),where Δ(0) is the refractive index delta at the centerline of the coremember.

Referring now to FIG. 1, a cross section of one embodiment of amulti-core optical fiber ribbon is schematically depicted. In oneembodiment, the multi-core optical fiber ribbon 100 generally comprisesat least two core members 102 surrounded by a single cladding layer 104.In another embodiment, the multi-core optical fiber ribbon comprises atleast three core members 102. The core members 102 are oriented inparallel with one another in a single plane. For example, in theembodiment of the multi-core optical fiber ribbon 100 depicted in FIG.1, the core members 102 are oriented such that each core member 102 isbisected by plane 108 which extends along a length of the multi-coreoptical fiber ribbon. In the embodiments described herein, the coremembers 102 are spaced apart from one another such that the cross-talkbetween adjacent core members is less than −25 dB, preferably less than−30 dB, even more preferably less than −35 dB and, most preferably, lessthan −40 dB. Cross-talk levels of less than −25 dB are generallyachieved by positioning the core members 102 such that thecenter-to-center spacing D between adjacent core members is ≧15 microns.In some embodiments, the core members 102 are equidistantly spaced suchthat the center-to-center spacing between core members is uniform acrossthe width W of the multi-core optical fiber ribbon 100.

Still referring to FIG. 1, the core members 102 are generally formedfrom silica-based glass and have a core index of refraction n_(c) and acore relative refractive index Δ_(c) % relative to the single claddinglayer 104. In the embodiments described herein, the silica-based glassof the core members 102 is doped with one or more dopants whichincreases the index of refraction of the core members 102. For example,the core members 102 may comprise silica-based glass doped withgermanium such as when the core members 102 comprise silica (SiO₂) glassup-doped with germania (GeO₂). However, it should be understood thatdopants other than germania may be utilized in the core members,including, without limitation, TiO₂, ZrO₂, Nb₂O₅ and/or Ta₂O₅. Suchdopants may be incorporated in the core members 102 either individuallyor in combination in order to obtain the desired core index ofrefraction n_(c) and relative refractive index Δ_(c) %. In theembodiments described herein, the core members 102 contain from about4.0 wt. % to about 40 wt. % GeO₂. For example, in one embodiment, thecore members 102 comprise from about 4.0 wt. % to about 6.5 wt. % GeO₂,more preferably from about 5.0 wt. % to about 6.0 wt. % GeO₂, and, mostpreferably, from about 5.2 wt. % to about 5.5 wt. % GeO₂, whichincreases the index of refraction n_(c) of the core members 102 relativeto undoped silica glass. In the embodiments described herein, therelative refractive index Δ_(c) % of the core members 102 relative tothe single cladding layer 104 is ≧0.2%, more preferably ≧0.3% and, mostpreferably, from about 0.2% to about 2%.

In some embodiments, the core members 102 have a step-index profile asdepicted in the refractive index profile (a) of FIG. 1. In otherembodiments, the core members 102 have a graded index as depicted in therefractive index profile (b) of FIG. 1. In still other embodiments, thecore members 102 may have an α-profile with an α-value which defines theindex of refraction of the core members 102 as a function of the radiusof the core member 102. In embodiments where the core members 102 haveα-profiles, the α-value of the α-profile may be in a range from about1.9 to about 2.2 as measured at 1300 nm. In embodiments where the coremembers 102 have a graded index and/or an α-profile, the core members102 have a relative refractive index percent Δ_(c) % relative to thesingle cladding layer 104 and a maximum relative refractive indexpercent Δ_(cMax) % of greater than 0.5% and less than 2.2%, preferablyat least 0.6%, more preferably at least 1.0%, even more preferably atleast 1.5% and, most preferably, 2.0%.

In some embodiments described herein, the core members 102 are singlemode cores and have diameters d_(c)≦15 microns, preferably in the rangefrom about 3 microns to about 10 microns, more preferably from about 6microns to about 9 microns, and most preferably, from about 7 microns toabout 8 microns. In some embodiments, the core members 102 may be singlemoded at wavelengths from about 1260 nm to about 1700 nm. Alternatively,the core members 102 may be single-moded at wavelengths from about 1500nm to about 1700 nm. In embodiments where the core members 102 aresingle mode cores, the center-to-center spacing D between adjacent coremembers is ≧15 microns.

In embodiments where the core members 102 of the multi-core opticalfiber ribbon 100 are single-moded, the ratio R of the core spacing D tothe core diameter d_(c) is ≦6, preferably in a range from about 2 to 6.

In other embodiments, the core members 102 are multi-mode cores and havediameters d_(c)>15 microns, preferably >15 microns and ≦65 microns, morepreferably from about 25 microns to about 50 microns, and even morepreferably from about 35 microns to about 50 microns. In someembodiments the multi-mode cores support the propagation of multiplemodes at wavelengths from about 830 nm to about 880 nm. Alternatively,the multi-mode cores may support propagation of multiple modes atwavelengths from about 1020 nm to about 1100 nm. In embodiments wherethe core members 102 are multi-mode cores, the center-to-center spacingD between adjacent core members is ≧35 microns. In these embodiments,the core members 102 have a bandwidth of ≧300 MHz, preferably ≧500MHz/km, more preferably ≧750 MHz/km, even more preferably ≧1 GHz/km,and, most preferably, ≧2 GHz/km. The multi-mode cores generally have agraded refractive index profile such as the refractive index profile (b)of FIG. 1. More specifically, the multi-mode cores generally have agraded index α profile with an α value from about 1.9 to about 2.1, asdescribed above.

In embodiments where the core members 102 are multi-moded, the ratio Rof the core spacing D to the core diameter d_(c) is in a range fromabout 1 to about 3, more preferably less than about 2.0 and, mostpreferably, less than about 1.5.

In some embodiments described herein the relative refractive index Δ_(c)% of adjacent cores members 102 are substantially the same such thatadjacent core members 102 are phase matched. However, in otherembodiments, the indices of refraction of adjacent core members aredifferent which, in turn, creates a variation in the relative refractiveindex Δ_(c) % of adjacent core members 102. This variation in therelative refractive index Δ_(c) % of adjacent core members is referredto herein as a delta variation. For a given center-to-center spacing D,a delta variation between adjacent core members 102 reduces thecross-talk between the adjacent core members 102. Accordingly, adjacentcore members 102 with a delta variation may be placed closer togetherthan adjacent core members which do not have a delta variation withoutincreasing the amount of cross-talk between the adjacent core members.In some embodiments described herein, adjacent core members 102 may havea delta variation ≧1% which allows the core members to be positionedcloser together without increasing the cross-talk between the coremembers. In some embodiments, the effective index variation betweenadjacent core members (i.e., the difference in the index of refractionof adjacent core members) is greater than or equal to about 5×10⁻⁵.

Referring now to FIGS. 2A and 2B, another embodiment of a multi-coreoptical fiber ribbon 100 is schematically depicted. In this embodimentthe optical fiber ribbon comprises a plurality of core members 102. Eachcore member 102 comprises a central core portion 181 surrounded by alow-index ring 182. The low-index ring 182 generally has an index ofrefraction n_(l) and a radial thickness r from about 5 microns to about20 microns. The index of refraction n_(l) of the low-index ring 182 issuch that n_(l)≦n_(cl)≦n_(c) which yields a refractive index profile asdepicted in (c) of FIG. 2. In some embodiments, the radial thickness ofthe low-index ring may be less than about 10 microns and, morepreferably, less than about 5 microns. The low index ring 182 maycomprise silica glass down-doped with fluorine. For example, the lowindex ring 182 may comprise from about 0.36 wt. % to about 3.6 wt. %fluorine, more preferably from about 0.72 wt. % to about 2.5 wt. %fluorine, and most preferably, from about 1.4 wt. % to about 2.5 wt. %fluorine such that the relative refractive index percent Δ_(l)% of thelow index ring 182 relative to the single cladding layer 104 is lessthan about −0.1%, more preferably less than about −0.4%, even morepreferably from about −0.4% to about −0.7%. For a given center-to-centerspacing D between adjacent core members 102, core members 102 havingcore portions 181 surrounded by low index rings 182 have reduced thecross-talk between adjacent core members. Thus, adjacent core members102 with low index rings 182 may be placed closer together than adjacentcore members which do not have low index rings without increasing theamount of cross-talk between the adjacent core members. Accordingly, insome embodiments described herein, core members with low index rings maybe utilized to decrease the spacing between adjacent core members.

While FIGS. 1 and 2A depict multi-core optical fiber ribbons with aplurality of core members oriented in parallel in a single plane, itshould be understood that, in other embodiments, the multi-core opticalfiber ribbon 100 may be formed with a plurality of core members orientedin different planes. Referring to FIG. 3 by way of example, a crosssection of one embodiment of a multi-core optical fiber ribbon 190 isschematically depicted. In this embodiment, the multi-core optical fiberribbon 190 comprises a plurality of core members 102 oriented inparallel with one another. However, in the embodiment of the multi-coreoptical fiber ribbon 190 shown in FIG. 3, the core members 102 areoriented in a plurality of parallel planes. For example, a first group191 of core members 102 are oriented such that the core members 102 arebisected by a first plane 192. A second group 193 of core members 102 isoriented such that the second group 193 of core members 102 is bisectedby a second plane 194 which is parallel with the first plane 192.Accordingly, it should be understood that the first group 191 of coremembers 102 is generally parallel to and non-coplanar with the secondgroup 193 of core members 102. Further, while FIG. 3 depicts amulti-core optical fiber ribbon 100 with two planes of core members 102,it should be understood that the multi-core optical fiber ribbonsdescribed herein may comprise a single plane of core members or three ormore planes of core members.

Referring again to FIG. 1, the core members 102 are surrounded by asingle cladding layer 104. In the embodiments described herein, thesingle cladding layer 104 is formed from silica-based glass (SiO₂) withan index of refraction n_(cl) which is less than the core index ofrefraction n_(c) (i.e., n_(cl)<n_(c)). In some embodiments the singlecladding layer is formed from pure silica-based glass without anydopants which change the index of refraction of silica, such asup-dopants (i.e., germanium and the like) or down-dopants (i.e., boron,fluorine and the like). In other embodiments, the single cladding layermay comprise one or more up-dopants which increase the refractive indexof the silica glass, or one or more down-dopants which decreases therefractive index of the silica glass, so long as the cladding index ofrefraction n_(cl) is less than the core index of refraction n_(c) andthe relative refractive index Δ_(c) % of the core members 102 relativeto the single cladding layer 104 is greater than about 0.2%, morepreferably ≧0.3% and, most preferably, from about 0.2% to about 2%, asdescribed above.

The single cladding layer 104 is generally rectangular in cross sectionwith a width W and a thickness T. In some embodiments, the width W maybe equal to the thickness T such as when the single cladding layer 104is square in cross section. The width W of the core portion is dependenton the number of core members 102 included in the multi-core opticalfiber ribbon 100. However, the thickness T of the multi-core opticalfiber ribbon 100 is such that the multi-core optical fiber ribbon 100 isflexible in the width W direction and may be coiled in the widthdirection to a radius of ≦140 mm, more preferably ≦75 mm and, mostpreferably, ≦5 mm without damaging the glass of the multi-core opticalfiber ribbon. In the embodiments described herein, the thickness T ofthe multi-core optical fiber ribbon 100 is ≦400 microns, more preferably≦200 microns, even more preferably ≦125 microns and, most preferably,from about 50 microns to about 125 microns.

Referring again to FIG. 2, in some embodiments the multi-core opticalfiber ribbon 100 may further comprise at least one optical coating layer120 which surrounds and directly contacts the single cladding layer 104.The optical coating layer 120 generally has a thickness T_(oc) fromabout 50 microns to about 150 microns. The optical coating layer has arefractive index n_(ct)≧the refractive index n_(cl) of the singlecladding layer 104. In the embodiment shown in FIG. 2, the opticalcoating layer 120 comprises a primary coating layer 122 and a secondarycoating layer 124. The primary coating layer 122 surrounds and directlycontacts the single cladding layer 104 and is formed of relatively softpolymer materials. The primary coating layer 122 has a thickness fromabout 25 microns to about 125 microns. The secondary coating layer 124is formed around and directly contacts the primary coating layer 122 andhas a thickness from about 50 microns to about 125 microns. Thesecondary coating layer 124 is generally formed from polymer materialswhich are relatively harder than the polymer materials from which theprimary coating layer 122 is formed. More specifically, the primarycoating layer 122 preferably exhibits a Young's modulus less than 100MPa, more preferably less than 50 MPa, and most preferably less than 10MPa while the secondary coating layer 124 preferably exhibits a Young'smodulus greater than 500 MPa, more preferably greater than 700 MPa, andmost preferably greater than 900 MPa. The materials used in the primaryand secondary coating layers are UV curable urethane acrylate coatingmaterials. For example, the primary and secondary coatings may comprisematerials similar to those disclosed in U.S. Pat. Nos. 6,849,333 and6,775,451.

While the embodiment of the multi-core optical fiber ribbon 100 of FIG.2 is depicted with an optical coating layer 120 which comprises aprimary coating layer 122 and a secondary coating layer 124, it shouldbe understood that, in other embodiments, the optical coating layer 120only comprises primary coating layer 122. Further, it should beunderstood that the optical coating layer 120 is optional and that, insome embodiments, the multi-core optical fiber ribbon 100 may be formedwithout an optical coating layer 120 as shown in FIG. 1.

In the embodiments of the multi-core optical fiber ribbons describedherein, the optical fiber ribbons may be formed in any length. However,it should be understood that the cross-talk between adjacent coremembers in the multi-core optical fiber ribbon decreases as the lengthof the optical fiber ribbon decreases. Accordingly, in some embodiments,the length of the multi-core optical fiber ribbon may be less than 500m, less than 250 m or even less than 100 m.

Methods for producing multi-core ribbon preforms from which multi-coreoptical fiber ribbon may be drawn will now be described with specificreference to FIGS. 4-11.

Referring now to FIGS. 4-6, one embodiment of a method for forming amulti-core ribbon preform by outside vapor deposition (OVD) isschematically depicted. In this embodiment, a core cane assembly 200 isfirst constructed. The core cane assembly 200 generally comprises aplurality of glass core canes 202 that are oriented in parallel with oneanother in a single plane such that the core canes 202 can be drawn intothe core members of the optical fiber ribbon described above. In theembodiment of the core cane assembly 200 shown in FIG. 4, the core canes202 are tacked together with glass attachment elements 204. The glassattachment elements 204 are generally formed from silica-based glasshaving the same composition as the single cladding layer of themulti-core optical fiber ribbon as these elements will ultimately becomea part of the single cladding layer. The glass attachment elementssecure the core canes 202 to one another, maintain the desired spacingbetween adjacent core canes, and keep the core canes substantiallyco-planar with one another. The construction of the core cane assemblyis completed by fusing glass rods 206 to both ends of the core caneassembly to support the core cane assembly on the spindles of an OVDlathe.

In an alternative embodiment (not shown) the individual core canes 202of the core cane assembly 200 may be joined together without the use ofthe glass attachment elements 204. For example, in one embodiment, thecore canes 202 are formed with a thin cladding layer which surrounds thecore preform portion of the core cane such that the core cane has acore:clad ratio from about 0.1 to about 0.5. The thin cladding layersurrounding the core canes may be utilized to maintain the desiredspacing between adjacent core canes as well as to attach adjacent corecanes together. For example, in one embodiment, the adjacent core canesare sintered together at the thin cladding layer to create the core caneassembly.

Referring now to FIG. 5, once the core cane assembly 200 is constructed,the core cane assembly 200 is positioned in an OVD lathe (not shown)such that the silica-based glass forming the single cladding preformlayer may be deposited on the core cane assembly 200. In order toachieve the substantially rectangular cross section of the preformlayer, a burner 211 is traversed over a first surface 208 of the corecane assembly as a flow of carrier gas G in which silica-glass precursormaterials are entrained is directed onto the core cane assembly. In theembodiments described herein, the carrier gas comprises a flow of O₂ andCH₄ in which vapor-phase silica glass precursor materials are mixed andpyrolyzed by the flame of the burner 211 to create silica-glass soot 212which is deposited on the first surface 208 of the core cane assembly200. Where the single cladding layer of the resultant multi-core opticalfiber ribbon does not contain a dopant, the vapor-phase silica glassprecursor material may be SiCl₄ and the pyrolysis process yieldssilica-glass soot which is deposited on the first surface 208 of thecore cane assembly. However, if the single cladding layer containsdopants, such as one or more vapor-phase dopants (e.g., an up-dopantsuch as GeO₂ or a similar down dopant such a B₂O₃), the vapor phasedopants may be combined with the SiCl₄ such that the pyrolysis processyields doped silica-glass soot which is, in turn, deposited on the firstsurface 208 of the core cane assembly 200. In the embodiments describedherein the temperature of the pyrolysis operation is approximately 1500°C.

In one embodiment, the burner 211 is traversed over the first surface208 of the core cane assembly 200 until the desired amount ofsilica-glass soot 212 is deposited on the first surface 208. Thereafter,the core cane assembly 200 is rotated and the pyrolysis process isrepeated on the second surface 210 of the core cane assembly 200 untilthe desired amount of silica-glass soot is deposited on the secondsurface 210 of the core cane assembly 200, as depicted in FIG. 6.Building up the soot on each of the first surface 208 and the secondsurface 210 separately permits the deposited silica-glass soot 212 tohave a substantially rectangular cross section, which, in turn, may besubsequently imparted to the multi-core optical fiber ribbon. In analternative embodiment, the core cane assembly 200 may be continuouslyrotated as the burner is repeatedly traversed along the axial length ofthe core cane assembly 200 such that the silica-glass soot 212 isdeposited on both the first surface 208 and the second surface 210 asthe core cane assembly 200 is rotated.

Reference has been made herein to depositing the soot on the firstsurface 208 and the second surface 210 of the core cane assembly 200.However, it should be understood that, as silica-glass soot is depositedon the first surface 208 and the second surface 210, silica-glass sootis also deposited on the edges of the core cane assembly 200 such that,after the silica glass soot is deposited, the core cane assembly 200 isencircled by a layer of silica-glass soot.

After the silica-glass soot has been deposited on the first surface 208and the second surface 210 of the core cane assembly 200, thesilica-glass soot 212 is consolidated on the core cane assembly todensify the glass soot and form a single cladding preform layer having asubstantially rectangular cross section around the core cane assembly.The silica-glass soot is consolidated by drying the silica-glass soot212 on the core cane assembly 200 in flowing chlorine gas at atemperature from about 1000° C. to about 1100° C. and then heating thesilica glass soot 212 on the core cane assembly 200 to a temperaturerange from about 1450° C. to about 1550° C. in a consolidation oven toproduce a single cladding preform layer of fully dense silica-basedglass having the desired composition around the core cane assembly 200.

Referring now to FIG. 7, in another embodiment, a multi-core opticalfiber preform is formed by compressing silica glass soot around a corecane assembly 200. In this embodiment a core cane assembly is positionedin a rectangular mold cavity 222 of a mold body 220. In one embodiment,the core canes 202 of the core cane assembly are individually positionedin the rectangular mold cavity 222. Each individual core cane 202 mayextend through an upper ram 224 and a lower ram 225 positioned in therectangular mold cavity 222, which, in turn, maintains the spacingbetween the core canes and the orientation of the core canes relative tothe mold body 220. In another embodiment, the core canes 202 of the corecane assembly 200 are attached together, as described hereinabove,before the core cane assembly is inserted into the mold cavity. Ingeneral, the core canes 202 of the core cane assembly are oriented inparallel with one another in a single plane, as described hereinabove.With the core cane assembly 200 positioned in the mold body 220,silica-glass soot 226 having the desired composition is loaded into therectangular mold cavity 222 around the core cane assembly 200.

Referring now to FIG. 8, the silica-glass soot is compressed to form asoot compact 227 around the core cane assembly 200. In the embodimentshown in FIG. 8, the silica-glass soot is compressed into a soot compact227 by advancing the rams 224, 225 towards one another along the axiallength of the core cane assembly 200. The rams 224, 225 may be advancedusing a hydraulic press, a mechanical press or any other press suitablefor exerting a force F on the rams 224,225. Further, it should beunderstood that the silica glass soot may be compressed by advancing oneof the rams (either the upper ram 224 or the lower ram 225) towards theother ram which is held stationary relative to the mold body 220. Thesilica-glass soot is compressed until the soot reaches a density fromabout 0.5 g/cc to about 1.2 g/cc, more preferably greater than about 0.7g/cc and less than about 1.1 g/cc, and most preferably greater thanabout 0.8 g/cc and less than about 1.0 g/cc. As the silica-glass soot iscompressed, the soot compact 227 takes on the rectangular shape of therectangular mold cavity.

While the embodiment shown in FIG. 8 depicts compressing thesilica-glass soot in an axial direction of the core cane assembly 200,it should be understood that, in alternative embodiments, thesilica-glass soot may be compressed in a radial direction to form thesoot compact 227 around the core cane assembly 200.

After the silica-glass soot is compressed into the soot compact 227, thecombination of the soot compact 227 and core cane assembly 200 isremoved from the mold body 220 and the soot compact 227 is consolidatedon the core cane assembly 200 using the techniques described above tobond the soot compact 227 to the core cane assembly as well as todensity the silica-glass soot thereby forming a multi-core ribbonpreform.

Referring now to FIG. 9, a multi-core ribbon preform 250 from which amulti-core optical fiber ribbon may be drawn is schematically depicted.The multi-core ribbon preform 250 generally comprises a core caneassembly 200 which is encircled by a single cladding preform layer 230formed by the consolidation of silica-glass soot on the core caneassembly 200. In the embodiments described herein, the single claddingpreform layer 230 is generally rectangular in cross section such thatoptical fiber ribbon drawn from the multi-core ribbon preform 250 isalso substantially rectangular in cross section. Moreover, while themulti-core ribbon preform 250 is shown with a core cane assemblycomprising a plurality of core canes oriented in parallel in a singleplane, it should be understood that, in alternative embodiments (notshown), the core cane assembly may be formed with multiple (i.e., 2 ormore) planes of core cane members, each of which is parallel andnon-coplanar with the other planes of core cane members.

Referring now to FIG. 10, in an alternative embodiment, a multi-coreribbon preform 250 may be formed using a stack and draw technique. Inthis embodiment a core cane assembly 200 is positioned in a rectangularglass cladding tube 260 such that the core canes 202 of the core caneassembly are oriented in parallel with one another in a single plane, asdescribed hereinabove. The rectangular glass cladding tube 260 is formedfrom silica-based glass having the same composition as is desired forthe single cladding layer of the multi-core optical fiber ribbon. Ingeneral, the silica-based glass of the rectangular glass cladding tube260 has an index of refraction n_(cl) which is less than the index ofrefraction n_(c) of the core canes 202. In the embodiments describedherein, the rectangular glass cladding tube 260 has a wall thickness ina range from about 50 mm to about 125 mm, more preferably less than 125mm, even more preferably less than 100 mm, and, most preferably, lessthan 90 mm. In one embodiment, the core canes 202 of the core caneassembly 200 are individually positioned in the rectangular glasscladding tube 260. In another embodiment, the core canes 202 of the corecane assembly 200 are attached together, as described hereinabove,before the core cane assembly 200 is inserted into the rectangular glasscladding tube 260. A plurality of filler canes 262 are positionedbetween the rectangular glass cladding tube 260 and the core caneassembly 200. In the embodiments described herein, the filler canes 262have the same composition and index of refraction as the rectangularglass cladding tube 260 (i.e., the filler cane index of refractionn_(filler)=the rectangular glass cladding tube index of refractionn_(cl)). In this embodiment of the multi-core ribbon preform 250, themulti-core ribbon preform is placed under vacuum as the multi-coreoptical fiber ribbon is drawn from the preform in order to collapse andseal the interstitial spaces between adjacent filler canes 262 and corecanes 202.

Referring to FIG. 11, one embodiment of a system 300 for producing amulti-core optical fiber ribbon from a multi-core ribbon preform isschematically illustrated. The system 300 generally comprises a drawfurnace 302 for heating a multi-core ribbon preform 250 such that amulti-core optical fiber ribbon 100 may be drawn from the multi-coreribbon preform 250. The draw furnace 302 is generally verticallyoriented such that multi-core optical fiber ribbon 100 drawn from thepreform 250 exits the furnace along a substantially vertical pathway(i.e., a pathway that is substantially parallel with the z-direction ofthe coordinate axes depicted in FIG. 11).

After the multi-core optical fiber ribbon exits the draw furnace 302,the dimensions of the multi-core optical fiber ribbon 100 and the drawtension applied to the multi-core optical fiber ribbon 100 are measuredwith non-contact sensors 304, 306. As shown in FIG. 11, after thediameter and tension of the multi-core optical fiber ribbon 100 ismeasured, the multi-core optical fiber ribbon 100 passes through acooling system 308 which cools the multi-core optical fiber ribbon toless than about 80° C. and, more preferably, less than about 60° C.

After the multi-core optical fiber ribbon exits the cooling system 308,the multi-core optical fiber ribbon enters a coating system 310 in whichan optical coating layer is applied to the multi-core optical fiberribbon. As the multi-core optical fiber ribbon 100 exits the coatingsystem 310, the dimensions of the multi-core optical fiber ribbon ismeasured again using a non-contact sensor 312. Thereafter, a non-contactflaw detector 314 is used to examine the multi-core optical fiber ribbonfor damage and/or flaws that may have occurred during manufacture. Afiber take-up mechanism 315 utilizes various drawing mechanisms 316 andpulleys 318 to provide the necessary tension to the multi-core opticalfiber ribbon 100 as the multi-core optical fiber ribbon is drawn throughthe system 300 and wound onto a storage spool 320.

EXAMPLES

Embodiments of the multi-core optical fiber ribbons will be furtherclarified by the following prophetic modeled examples. In each modeledexample the multi-core optical fiber ribbon consisted of a pair of coremembers positioned in a single cladding layer. The cross-talk for eachmodeled multi-core optical fiber ribbon was determined utilizing thefollowing equations. The powers transmitted in two adjacent core membersmay be calculated as:

${P_{1} = {{\cos^{2}({gz})} + {\left( \frac{\Delta\beta}{2g} \right)^{2}{\sin^{2}({gz})}}}},{and}$${P_{2} = {\left( \frac{\kappa}{g} \right)^{2}{\sin^{2}({gz})}}},$where z is the length of the fiber, κ is the coupling coefficient, Δβ isthe mismatch propagation constant between the modes propagating in theadjacent core members when they are insulated from one another, and g isa parameter depending on κ and Δβ such that

$g^{2} = {\kappa^{2} + {\left( \frac{\Delta\beta}{2} \right)^{2}.}}$The cross-talk can be calculated based on the power transmitted throughthe adjacent core members utilizing the following relationship:

$X = {{10{\log\left( \frac{P_{2}}{P_{1}} \right)}} = {10{{\log\left( \frac{4\kappa^{2}}{{4g^{2}c\;{\tan({gz})}} + ({\Delta\beta})^{2}} \right)}.}}}$

Example 1

Referring to FIG. 12, a pair of multi-core optical fiber ribbons weremathematically modeled. Each core member of the first optical fiberribbon was modeled with a step refractive index profile and a corerelative refractive index Δ_(c) % of about 0.34%. Each core member had adiameter d_(c) of 8.4 microns and was single-moded at a wavelength of1550 nm. The core members were phase matched such that the deltavariation between the adjacent core members was zero. Curve 402 in FIG.12 indicates the cross-talk of the first, phase matched optical fiberribbon as a function of the distance between the adjacent core membersfor a fiber length of 100 m at a wavelength of 1550 nm. As shown in FIG.12, a cross-talk of less than −35 dB is only achieved when the spacingbetween the two core members is less than 54 microns.

Still referring to FIG. 12, the second multi-core optical fiber ribbonwas modeled with a step index refractive index with each core memberhaving a diameter d_(c) of 8.4 microns such that the core members weresingle-moded at a wavelength of 1550 nm. However, in this embodiment,the core members had a delta variation of approximately 1% and thecorresponding effective index change of the fundamental mode propagatedin each fiber is about 5×10⁻⁵. As shown in FIG. 12, a cross-talk of lessthan −35 db is achievable when the spacing between the two core membersis 28 microns. The effective index variation can also be achieved byvarying the core radius. In the following hypothetical examples thecross-talk was calculated assuming the adjacent core members had aneffective index variation of approximately 5×10⁻⁵.

Example 2

Referring now to FIG. 13, a pair of multi-core optical fiber ribbonswere mathematically modeled. Each core member of both optical fiberribbons was modeled with a step refractive index profile and a corerelative refractive index Δ_(c) % of about 0.34%. Each core member had adiameter dc of 8.4 microns and was single-moded at a wavelength of 1550nm. However, in this example, the first optical fiber ribbon had alength of 100 meters while the second optical fiber ribbon had a lengthof 2 meters. Curve 406 shows the cross-talk as a function of thecenter-to-center spacing of adjacent core members for the 2 meter ribbonwhile curve 408 shows the cross talk as function of the center-to-centerspacing of adjacent core members for the 100 meter ribbon. As shown inFIG. 13, the 2 meter ribbon generally had a cross-talk which was about 3dB less than the cross-talk of the 100 m ribbon.

Example 3

Referring to FIG. 14, a pair of multi-core optical fiber ribbons weremathematically modeled. Each core member of the first optical fiber wasmodeled with a step refractive index profile and a core relativerefractive index Δ_(c) % of about 0.34%. Each core member had a diameterd_(c) of 8.4 microns and was single-moded at a wavelength of 1550 nmEach core member of the second optical fiber was modeled with a steprefractive index profile and a core relative refractive index Δ_(c) % ofabout 1.0%. Each core member had a diameter d_(c) of 4.9 microns and wassingle-moded at a wavelength of 1550 nm Curve 410 shows the cross-talkas a function of the center-to-center spacing of adjacent core membersfor the first multi-core ribbon while curve 412 shows the cross talk asfunction of the center-to-center spacing of the second multi-coreribbon. As shown in FIG. 13, increasing the relative refractive indexΔ_(c) % of the core members and decreasing the core diameter reduces thecross-talk between the core members as the power of each core member ismore confined to that core member which, in turn, reduces the couplingcoefficient between the adjacent core members.

Example 4

Referring to FIG. 15, a pair of multi-core optical fiber ribbons weremathematically modeled. Each core member of both optical fiber ribbonswas modeled with a step refractive index profile and a core relativerefractive index Δ_(c) % of about 0.34%. Each core member had a diameterd_(c) of 8.4 microns and was single-moded at a wavelength of 1550 nm.However, in this example, the core members of the second fiber ribbonwere formed with a low-index ring as the outermost portion of each coremember. The low-index ring had a relative refractive index Δ_(l) % of−0.4% and a radial thickness of 4 microns. Curve 414 shows thecross-talk as a function of the center-to-center spacing of adjacentcore members for the ribbon without the low-index rings and curve 416shows the cross talk as function of the center-to-center spacing ofadjacent core members for the ribbon formed with the low index rings. Asdemonstrated in FIG. 15, the fiber ribbon with the low index rings canachieve a −35 db cross-talk with a smaller center-to-center spacing thanthe ribbon formed without the low index rings.

Example 5

In this example, a multi-mode optical fiber ribbon was modeled. Eachcore member had a relative refractive index Δ_(c) % of 1.0% and adiameter d_(c) of 50 microns. Each core member supported 18 mode groupsat 850 nm. In order for the fiber to achieve a cross-talk of −35 dB atan operating wavelength of 850 nm under overfilled launch conditions, itwas determined that the center-to-center spacing between adjacent coresshould be at least 66 microns.

Example 6

In this example, a multi-mode optical fiber ribbon was modeled. Eachcore member had a relative refractive index Δ_(c) % of 2.0% and adiameter d_(c) of 25 microns. Each core member supported 13 mode groupsat 850 nm. In order for the fiber to achieve a cross-talk of −35 dB atan operating wavelength of 850 nm under overfilled launch conditions, itwas determined that the center-to-center spacing between adjacent coresshould be at least 41 microns.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the embodiments describedherein without departing from the spirit and scope of the claimedsubject matter. Thus it is intended that the specification cover themodifications and variations of the various embodiments described hereinprovided such modification and variations come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method for forming a multi-core optical fiberribbon, the method comprising: forming a core cane assembly comprising aplurality of core cane members oriented in parallel in a single plane;positioning the core cane assembly in a flow of carrier gas comprisingsilica-glass precursor materials; traversing a burner across a firstsurface of the core cane assembly to react the silica-glass precursormaterials in the flow of carrier gas thereby causing silica-glass sootto be deposited on the first surface of the core cane assembly, whereinthe plurality of core cane members have a relative refractive indexΔ_(c) % from about 0.2% to about 2.0% relative to the silica-glass soot;traversing the burner across a second surface of the core cane assemblyto react the silica-glass precursor materials in the flow of carrier gasthereby causing silica-glass soot to be deposited on the second surfaceof the core cane assembly; consolidating the silica-glass soot on thecore cane assembly to form a multi-core ribbon preform comprising thecore cane assembly encircled by a single cladding preform layer, whereinthe single cladding preform layer is substantially rectangular in crosssection; and drawing the multi-core ribbon preform into the multi-coreoptical fiber ribbon.
 2. The method of claim 1, wherein the core caneassembly comprises a plurality of glass attachment elements coupling theplurality of core cane members to one another.
 3. The method of claim 1,wherein: each of the plurality of core cane members comprises a claddinglayer such that each of the plurality of core cane members have acore:clad ratio from about 0.1 to about 0.5; and the plurality of corecane members are sintered together to form the core cane assembly. 4.The method of claim 1, wherein the plurality of core cane members of thecore cane assembly are equidistantly spaced.
 5. The method of claim 1further comprising rotating the core cane assembly in the flow ofcarrier gas as the burner is traversed across the first surface and thesecond surface.
 6. The method of claim 1, wherein each of the pluralityof core cane members comprise an outer layer of silica-glass soot andthe plurality of core cane members are sintered together to form thecore cane assembly.
 7. The method of claim 1, wherein the core caneassembly comprises glass rods fused to ends of the core cane assembly.8. The method of claim 1, wherein the plurality of core cane memberscomprise silica glass up-doped with from about 4.0 wt. % to about 40 wt.% GeO₂ and a delta variation between adjacent core cane members is ≧1%.9. The method of claim 1, wherein adjacent glass core cane members arenon-phase matched.
 10. The method of claim 1, wherein the plurality ofcore cane members comprise an outer layer of silica-glass soot and theplurality of core cane members are sintered together.